The American cover art for Fire and Blood
The book was originally written across three or four months back in 2012 as part of The World of Ice and Fire, but Martin ended up producing way too much material for that book, leaving his co-authors Linda Antonsson and Elio Garcia, Jr. to summarise this material. This is the full, unexpurgated text and incorporates the sections previously published separately as The Sons of the Dragon, The Rogue Prince and The Princess and the Queen.
Martin's plan originally had been to hold back the entire history (including the as-yet-unwritten second half, covering Aegon III's reign through Blackfyre Rebellion, the War of the Ninepenny Kings and the Mad King's downfall in the War of the Usurper, aka Robert's Rebellion) until after A Song of Ice and Fire was completed. However, the material he'd already completed for the first half runs almost as long as A Game of Thrones itself, 989 manuscript pages and roughly 270,000-odd words (compared to AGoT's 1,088 manuscript pages and 298,000 words), so the work would have been published in two volumes anyway. Martin has no plans to write the second book as yet since it would by definition include major spoilers for the Dunk & Egg stories and also for A Song of Ice and Fire itself.
The (working) UK cover art for Fire and Blood.
Fire and Blood will be published on 20 November this year and will feature 75 black-and-white illustrations by Doug Wheately.